Afghan and coalition troops found and destroyed nearly 2.3 tonnes of marijuana in an abandoned school in southern Afghanistan, while coalition troops killed four militants elsewhere in the south, officials said Sunday.

The marijuana, which was stored in 0.6-metre tall stacks, filled several rooms of a school in the Arghistan district of the southern province of Kandahar, a statement from U.S. forces in Afghanistan said.

"No students or faculty were at the schoolhouse at the time of the discovery. The school's furniture had been taken out of the classrooms and left in the courtyard. The amount of rust on the furniture indicated the school may not have been used for its intended purpose for a prolonged period of time," the statement said.

Col. Jerry O'Hara, a U.S. military spokesman, said using a school as a drug warehouse "is an attack on the future of all Afghanistan."

Afghanistan is the world's largest producer of opium, the main ingredient in heroin. But as the U.S. and other Western nations have tried to help Afghanistan stamp out its poppy fields, an increasing number of farmers have turned to marijuana, which is receiving less attention from authorities.

In Zabul province, meanwhile, coalition forces killed four armed militants and detained five suspects during an operation Saturday, the coalition said.

The operation targeted a Taliban militant "known to traffic weapons and coordinate roadside bomb attacks," the military said.

Violence has spiked in Afghanistan the last two years, and some 65,000 international troops are now in the country.

Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, said Saturday during a trip to Kabul that the United States would send between 20,000 and 30,000 more forces to Afghanistan by summer. Those forces will primarily move into the country's south, where the insurgency is the most entrenched.